Narrative
At about 0740 on 7 October 2014, a Robinson R44 II helicopter registered ZK-HBQ departed Karamea aerodrome for Nelson, where it was due for scheduled maintenance. The pilot had delayed departure due to forecast high winds but decided to proceed after the owner of the helicopter reported wind speeds of 15-20 knots at Grange Ridge. The pilot flew up the Karamea River, then the Leslie River, and crossed the Tasman Ranges to the northeast of Mt Arthur at approximately 350 feet above ground level. The final position report from the flight tracking system at 0806 showed the helicopter at an altitude of 3,990 feet, groundspeed 83 knots. The accident occurred at about 0808.
After the helicopter failed to arrive at Nelson, the owner, who had departed Karamea later, reported encountering moderate turbulence near the Flora Saddle. He searched the area but did not find the wreckage. The accident site was located on 10 October 2014 after a four-day search.
Findings
The investigation determined that the helicopter broke up in flight after one of the main rotor blades struck the cabin and the main rotor assembly separated. The cause of this was mast-bumping, a phenomenon where excessive teetering of the main rotor causes the blade spindle to contact the main rotor driveshaft. Mast-bumping can occur when a helicopter enters a low-gravity (low-G) condition, caused by turbulence or flight control inputs.
The Commission found it very likely that the helicopter entered a low-G condition after crossing the Tasman Ranges downwind from Mt Arthur. Turbulence from a strong southwest wind likely included severe downdraughts that prolonged the low-G condition, initiating the mast-bumping. The Commission could not rule out a flight control input as having contributed.
Additional findings noted that if a previous wind limitation for inexperienced R44 pilots had remained in place, the pilot would have been prohibited from flying due to forecast strong winds. All Robinson helicopter models are susceptible to low-G mast-bumping. During prolonged low-G, Robinson helicopters can roll rapidly to the right, likely breaking up before recovery. The pilot's instinctive reaction to an unexpected right roll or unintentional limb movement during turbulence could lead to mast-bumping. The only approved recovery technique is applying aft cyclic as soon as low-G is felt.
Recommendations
The Commission recommended that the Federal Aviation Administration extend the limitations of AD 95-26-04 (currently applying to R22 models) to R44 and R66 models, and make them applicable to all pilots regardless of experience. It also recommended that the Director of Civil Aviation of New Zealand apply those limitations to R44 and R66 helicopters in New Zealand until the FAA actions the recommendation.
Key Lessons Identified
The investigation highlighted that pilots of two-bladed, semi-rigid rotor helicopters must be aware of the risks of moderate or greater turbulence in strong winds, especially in the lee of high terrain. If such turbulence is encountered, landing and waiting for improvement should be considered. Pilots should also be aware of increased susceptibility to low-G conditions when lightly loaded, and the effects of high power and high tail rotor on roll rate. For Robinson helicopters, the approved low-G recovery technique should be used immediately.